1. Field of the Invention
The present invention relates generally to AC drive power conversion systems and, more particularly, to a substantially zero rotation and substantially zero torque detector and method for an AC electric motor drive system for producing an idle control signal when the drive system is in the substantially zero rotation and substantially zero torque mode.
2. Prior Art
Direct current (DC) motors have traditionally been used in electric drive systems to produce a mechanical rotation over a variable rotation range at substantial torque levels. However, DC motors exhibit several major deficiencies, including high maintenance costs and radio frequency interference problems caused by arcing and concomitant mechanical deterioration of the brushes used in such motors.
The trend in recent years has been to use AC motors in electric drive systems which produce variable mechanical rotation of substantial torque. AC motors are attractive technically and commercially because of their lack of brushes and inherent ruggedness of design.
An excellent analysis of the theory and operation as well as the attributes and deficiencies of DC and AC motor types is found in Electrical Machinery, the Processes, Devices and Systems of Electromechanical Energy Conversion, 3d Ed., by A. E. Fitzgerald et al., McGraw-Hill Book Company, New York, 1971.
One type of AC motor is the AC induction motor. The AC induction motor has been used in AC drive power systems for producing a variable mechanical rotation of substantial torque.
In such drive systems, the AC induction motor produces a variable mechanical rotation of variable torque in response to an output signal, such as a current, of variable magnitude and frequency. This drive current is typically supplied from a variable frequency inverter. The inverter converts a DC current of controllable magnitude into the drive current of variable magnitude and frequency; in the case of the thyristor inverter, the drive current is generated as a result of the controlled gating of the thyristors. The inverter typically has commutating capacitors used to commutate automatically the thyristors. This automatic commutation produced by the commutating capacitors requires a charge of appropriate magnitude and polarity on each commutating capacitor.
The DC current of controllable magnitude provided to the inverter can be supplied from any DC current source, but typically is furnished by a DC converter via a DC link having an inductor.
A conventional drive system utilizing an AC induction motor typically can provide substantially zero rotation at substantial torque. One way this can be accomplished is by operating the induction motor in a "constant slip" mode. It is sufficient for present to state that per-unit slip s is expressed as s=(n.sub.1 -n)/n.sub.b, where n is the rotation produced by the rotor of the motor in revolutions per minute (rpm), n.sub.1 is the synchronous speed of the stator field of the motor in rpm, and n.sub.b is the synchronous speed of the stator field at the motor rated rpm. In this regard, reference is made to pages 188-189 of the Fitzgerald, et al reference presented above. As is apparent, the per-unit slip increases towards the maximum value of 1.0 as the rotation of the rotor decreases with respect to the synchronous speed of the stator field.
In addition to the substantially zero rotation at substantial torque mode of operation, it is desirable for the drive system utilizing an AC induction motor to be able to provide substantially zero rotation at substantially zero torque. Most AC motor control schemes fail to operate properly at this latter condition. One approach to accomplish this latter mode of operation is for the drive system to force the frequency of the drive current to a substantially zero value and to force its magnitude to a predetermined lower level. This approach is described in detail in U.S. patent application Ser. No. 032,856, "Apparatus and Method For Providing A Signal Porportioned to Flux In An AC Motor For Control of Motor Drive Current", by John H. Cutler and Loren H. Walker, filed the same day as the present invention, assigned to the assignee of the present invention and incorporated herein by reference. Other approaches can be used for causing the AC induction motor to provide substantially zero rotation and substantially zero torque.
In order to utilize the approach as is set forth in application Ser. No. 032,856 referenced above, it is desirable to develop an idle signal when substantially zero rotation and substantially zero torque exit. Such an idle control signal could be used in improved drive systems to effect control of desired system parameters and functions such as switching to some means for determining motor flux other than that utilized during motor rotation. Consequently, it would be advantageous for an idle control signal to be generated when a drive system is in a substantially zero rotation and substantially zero torque state.